Wet Snow Derived from Sentinel-1 as Indicator for Discharge

This animation shows the temporal changes in wet snow and snow cover as detected by the two approaches developed by Eurac Research, which use Sentinel-1 C-band SAR and MODIS instruments, respectively.

The investigated area corresponds to the upper Adige catchment closed at Bronzolo/Branzoll, in the Autonomous Province of Bolzano/South Tyrol, Northern Italy. This is an alpine watershed with a drainage area of approximately 7400 km2, and elevations ranging from about 200 m a.s.l. at the southern valley bottoms, to around 3900 m a.s.l. in the western upper ranges. The Adige, which is the second largest Italian river, is monitored within the South Tyrol border by hydrographic stations managed by the Province’s Hydrographic Office who kindly provided the discharge data plotted in the bottom part of the animation.

The multi-temporal maps, which are computed at the time frequency of Sentinel-1 acquisitions, are made by three overlapping layers:

the wet snow maps represented in blue

the snow maps represented in white

the hill-shade of the investigated area as seen by the SAR sensor in grayscale

In detail, for what concern the wet snow detection, we developed a classification method that combines the algorithm proposed in Nagler et al., 2016 and a machine learning approach which fully exploits the multi-temporal information carried by the high revisit time of Sentinel-1. The MODIS snow maps are instead generated with the algorithm described in Notarnicola et al., 2013.

Analysing the map

From the animation is possible to notice that at the end of March 2015 most of the snow that covers the catchment is dry. All the snow cover detected by the MODIS sensor is in fact not visible from the Sentinel-1 SAR instrument, from which the wet snow is detected only. In this condition, the river discharge is low, as it shown in the graph below the map.

While advancing during the season, the snow starts to melt, the wet snow area increases and the total snow cover area decreases. Consequently, the water discharge increases until the end of June, when the discharge begins to decrease because most of the water resource stored in the snow has been already depleted.